Dual photorecording on cholesteric azobenzene-containing LC polymer films using helix pitch phototuning and holographic grating recording† Alexander Ryabchun, * a Alexey Bobrovsky, a Anna Sobolewska, bc Valery Shibaev a and Joachim Stumpe b Received 14th December 2011, Accepted 22nd January 2012 DOI: 10.1039/c2jm16591k For the first time the possibility to perform dual recording in cholesteric LC polymer films was demonstrated by means of helix pitch photo-tuning and holographic grating recording, as far as these two photo-processes take place under the exposure to light with different wavelengths. For this purpose the cholesteric mixture of the nematic azobenzene-containing copolymer doped with the chiral- photochromic dopant was prepared and the photo-optical properties of thin planarly-oriented films of this mixture were investigated. It was found that the holographic grating recording over 1 min allows high diffraction efficiency values to be reached but does not disrupt the supramolecular helical structure that makes possible a two independent images recording. Introduction In recent decades much attention of scientists has been focused on studying holographic recording and the elaboration and improvement of new materials that can be used as holographic media in particular. Holographic information recording is interesting for its high density, as well as the ability to record 3D images. Among all the materials used to date for holog- raphy, polymers and polymer composites containing azo- benzene fragments can be separated into an individual group. 1–4 Also within this group one can single out specific liquid crys- talline (LC) azobenzene-containing polymer systems. LC poly- mers are characterized by the following features: ability of self- organization (in a certain temperature range), high optical anisotropy, cooperative effects and the capability of their properties to be controlled under the action of external fields. There are also some reports on grating recording in such polymers. The high surface relief modulation and/or refractive index modulation takes place during the phase-type hologram recording. The appearance of the surface relief grating (SRG) is usually attributed to polymer chain migration, 5,6 whereas the refractive index modulation is connected with either photoin- duced isotropization or molecular orientation and reor- ientation. 7,8 The first holographic studies using azobenzene- containing LC polymers, performed by Wendorff et al., 9–11 revealed a grating recording with high diffraction efficiency. A great deal of such studies have been accomplished on azo- benzene-containing LC polymers of nematic and smectic types, 12–16 but there are only a few works devoted to hologram recording in photochromic LC polymer systems of a cholesteric type. 17–20 In these studies relatively thick films (>10 mm) of cholesteric polymer were mainly investigated. The results of the investigations demonstrated the lower diffraction efficiency of the recorded gratings when compared to the similar nematic polymers, 20 although the values of diffraction efficiency could reach about 40%. 17 In the glassy state of cholesteric systems recorded holograms are fairly stable, but when they were heated above T g the holograms are erased faster in comparison with nematic systems. Generally, azobenzene-containing LC poly- mers of a cholesteric type can be successively used as media for holographic information storage. It should be mentioned that the process of grating recording in polymers with a cholesteric mesophase is often considered separately from processes occurring in the cholesteric helix on the supramolecular level. In this paper for the first time we have tried to fully exploit the specific optical properties of the thin photochromic cholesteric LC polymer films (thickness <5 mm) obtained by spin-coating. One of the most interesting optical properties of the cholesteric mesophase is their ability of the supramolecular helical structure to selectively reflect light with specific characteristics (wavelength and polarization). This peculiarity is responsible for creating a color image visible in the reflected light. 21 Another important feature of the considered systems results from the ability to control the orientation of photoactive (azobenzene-containing) fragments by UV or visible light illumination. 22,23 It is note- worthy that the photoinduced orientation and reorientation processes have a cooperative character, i.e., if, besides photo- chromic groups, the LC polymer also contains non-photo- chromic mesogenic groups the cooperative orientation of all the side groups takes place during the exposure. As mentioned a Faculty of Chemistry, Moscow State University, Lenin Hills 1, Moscow, 119991, Russia b Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, 14476 Potsdam, Germany c Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2jm16591k This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 6245–6250 | 6245 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 6245 www.rsc.org/materials PAPER Downloaded by Lomonosov Moscow State University on 07 March 2012 Published on 20 February 2012 on http://pubs.rsc.org | doi:10.1039/C2JM16591K View Online / Journal Homepage / Table of Contents for this issue